Most present day systems require a dedicated electrical contact in order to be powered from an external power supply. However, this tends to be impractical and requires the user to physically insert connectors or otherwise establish a physical electrical contact. Typically, power requirements also differ significantly, and currently most devices are provided with their own dedicated power supply resulting in a typical user having a large number of different power supplies with each power supply being dedicated to a specific device. Although, the use of internal batteries may avoid the need for a wired connection to a power supply during use, this only provides a partial solution as the batteries will need recharging (or replacing). The use of batteries may also add substantially to the weight and potentially cost and size of the devices.
In order to provide a significantly improved user experience, it has been proposed to use a wireless power supply wherein power is inductively transferred from a transmitter coil in a power transmitter device to a receiver coil in the individual devices.
Power transmission via magnetic induction is a well-known concept, mostly applied in transformers having a tight coupling between a primary transmitter coil and a secondary receiver coil. By separating the primary transmitter coil and the secondary receiver coil between two devices, wireless power transfer between these becomes possible based on the principle of a loosely coupled transformer.
Such an arrangement allows a wireless power transfer to the device without requiring any wires or physical electrical connections to be made. Indeed, it may simply allow a device to be placed adjacent to, or on top of, the transmitter coil in order to be recharged or powered externally. For example, power transmitter devices may be arranged with a horizontal surface on which a device can simply be placed in order to be powered.
Furthermore, such wireless power transfer arrangements may advantageously be designed such that the power transmitter device can be used with a range of power receiver devices. In particular, a wireless power transfer approach known as the Qi Specifications has been defined and is currently being developed further. This approach allows power transmitter devices that meet the Qi Specifications to be used with power receiver devices that also meet the Qi Specifications without these having to be from the same manufacturer or having to be dedicated to each other. The Qi standard further includes some functionality for allowing the operation to be adapted to the specific power receiver device (e.g. dependent on the specific power drain).
The Qi Specification is developed by the Wireless Power Consortium and more information can e.g. be found on their website: http://www.wirelesspowerconsortium.com/index.html, where in particular the defined Specification documents can be found.
Many wireless power transmission systems, such as e.g. Qi, supports communication from the power receiver to the power transmitter thereby enabling the power receiver to provide information to the power transmitter that may allow this to adapt to the specific power receiver or the specific conditions experienced by the power receiver.
In many systems, such communication is by load modulation of the power transfer signal. Specifically, the communication is achieved by the power receiver performing load modulation wherein a load applied to the secondary receiver coil by the power receiver is varied to provide a modulation of the power signal. The resulting changes in the electrical characteristics (e.g. variations in the current of the transmitter coil) can be detected and decoded (demodulated) by the power transmitter.
Thus, at the physical layer, the communication channel from power receiver to the power transmitter uses the power signal as a data carrier. The power receiver modulates a load which can be detected by a change in the amplitude and/or phase of the transmitter coil current or voltage.
More information of the application of load modulation in Qi can e.g. be found in chapter 6 of part 1 of the Qi wireless power specification (version 1.0).
Wireless power transmitters constructed according to the Qi v1.1 specification operate in the so-called inductive regime. In this mode, power transfer occurs at tight coupling (coupling factor typically above 0.3) with relatively high efficiency. If a larger distance (“Z-distance”) or more positioning freedom of the receiver is desired, power transfer typically occurs in the so-called resonant regime or mode with loose coupling (coupling factor typically below 0.3). In the resonant mode, the resonance frequencies of power transfer resonance circuits at the power transmitter and at the power receiver should match to achieve the maximum efficiency. Furthermore, it is often desirable for the drive frequency and the transmitter resonance frequency to be the same as this may reduce intermodulation effects between these.
An example of a wireless power transfer system using load modulation communication techniques is provided in WO2014/083015A1.
In many power transfer approaches, such as the Qi Specification, the power transmitter is arranged to adjust the current through the power transmitter coil in response to control data that it receives from the power receiver. Thus, the current is increased if the power receiver requests more power and is reduced if less power is requested.
In order to provide suitable dynamic characteristics, the control is typically implemented by an outer loop involving the messages from the power receiver. This outer loop sets a reference current level for the transmit coil current in response to the messages from the power receiver. The power transmitter then implements an inner loop which controls the measured current through the transmitter coil to match the reference current set by the outer loop.
The current is specifically adjusted by controlling one or more of: the operating frequency of the power signal (the drive frequency of the drive signal to the resonance circuit of the power transmitter), the rail voltage of the driver stage of the transmitter (and thus the voltage amplitude of the drive signal), and the duty cycle of the power signal/drive signal.
However, there is inherently a contradiction between the desire to keep the transmit coil current constant in order to control the power transfer and the use of load modulation. Indeed, the purpose of the current loop can be considered to be to keep the transmit coil current constant when load variations occur whereas load modulation is based on detecting transmit coil current variations caused by load variations. In systems, such as Qi, this conflict is resolved by applying a time division approach. Specifically, the control loop is only active for a short period of time with the rest of the time being available for communication by load modulation. Specifically, the control loop is only active for approximately 10 msec after each message is received from the power receiver, which is typically with an interval of 250 msec.
However, such an approach may be suboptimal in some scenarios. In particular, the time division may result in suboptimal and potentially slow power transfer control. This may in some scenarios result in an inappropriate power setting which could result in suboptimal or unacceptable performance of the wireless power transfer system. It may also complicate communication as this is restricted to times during which the power control is not active.
An improved power transfer approach would accordingly be advantageous. In particular, an approach that allows improved operation, improved power transfer, increased flexibility, facilitated implementation, facilitated operation, improved communication, improved power control, and/or improved performance would be advantageous.